The Her2/neu tumor antigen, encoded by the erbB2/neu protooncogene, is a 185 kDa protein that belongs to the human epidermal growth factor receptor family. It consists of a cysteine-rich extracellular domain (ECD, from amino acids 23 to 652) with several glycosylation sites, a hydrophobic transmembrane domain (from amino acids 653 to 675) and an intracellular tyrosine kinase domain (from amino acids 676 to 1255). The Her2/neu receptor is expressed on the cell membrane of a variety of epithelial cell types and regulates aspects of cell growth and division through binding of specific growth factors.
Her2/neu is expressed at low levels in many normal cells, but is over-expressed in a variety of cancers, including breast, ovarian, endometrial, gastric, pancreatic, prostate and salivary gland cancers. For example, approximately 30% of metastatic breast cancers have been shown to over-express Her2/neu. This over-expression is associated with a poor prognosis for the breast cancer patient, as it corresponds to decreased relapse-free periods and shortened survival time. Currently, the most common forms of treating breast cancer involve surgery, chemical intervention and/or radiotherapy. Unless the cancer is restricted to a defined area, surgery alone cannot eliminate the cancer. Moreover, radiation treatment as well as chemotherapy may entail severe negative side effects.
In view of the disadvantages associated with the current therapies, attempts have been made to find additional approaches for treating proliferative disorders such as breast cancer, including immunotherapy.
The clinical implications of Her2/neu over-expression in tumors have made Her2/neu an attractive target for antibody-mediated immunotherapy, alone or as an adjunct to conventional chemotherapy. For example, the monoclonal antibody (mAb) 4D5 has been shown to reduce the growth of Her2/neu expressing tumours in mice by direct and indirect mechanisms such as apoptosis, antibody-dependent cell-mediated cytotoxicity (ADCC) or complement-dependent cytotoxicity (CDC). Based on these results, a humanized form of this antibody, Trastuzumab (Herceptin®), was tested in clinical trials. Increased overall survival of patients with breast tumors overexpressing Her2/neu was observed following cytotoxic treatment plus Herceptin®, as compared to chemotherapy or Trastuzumab alone. Herceptin® is now used as monotherapy but shows even higher efficacy in combination with cytotoxic chemotherapy. It is to be noted, however, that Trastuzumab is generally only effective in breast cancer where the Her2/neu receptor is over-expressed. Furthermore, multiple infusions are typically required, resulting in high treatment costs.
An alternative approach to the treatment or prevention of Her2/neu-associated cancers using passive immunotherapy with monoclonal antibodies such as Trastuzumab is based on the induction of tumour-specific humoral and/or cellular immune responses and the identification of antigens recognized by human B- and T-lymphocytes. For example, numerous antibodies directed against the extracellular domain (ECD) of Her2/neu have been generated by immunizing mice with cells expressing Her2/neu. The biological effect of these antibodies appears to be epitope-specific; that is, it is based on specific recognition of a short subsequence within the Her2/neu ECD. However, some antibodies have no effect or even actively stimulate tumour growth.
Such vaccine immunotherapy for cancer has been based on antigens against which humoral and/or cellular responses are elicited. These antigens should ideally be expressed or over-expressed exclusively by tumour cells, often referred to as tumour-associated antigens (TAAs). One of the first TAAs described for breast cancer was Her2/neu. Meanwhile, various TAAs representing different epitopes have been tested, but so far none have successfully made their way into clinical practice.
Depending on the type of immune response intended (i.e., B cell or T cell response), different strategies are typically applied. For instance, to induce a B cell (i.e. antibody) response, the antigens should comprise a B cell epitope. As generally understood in the art, a B cell epitope is a part of an antigen that is recognized and bound by a B cell receptor. Lipids, polysaccharides and proteins/peptides may contain B cell epitopes which, upon introduction into an organism of choice, cause B cells to produce antibodies which specifically bind to the introduced epitope.
Individual fragments of the ECD of Her2/neu, including B cell epitopes, are known in the art. For example, WO 2002/068474 describes a vaccine that comprises a peptide of 9-25 amino acids which sequence occurs in the extracellular part of the Her2/neu protein. Further, WO 2007/118660 describes a multi-peptide vaccine comprising a specific combination of peptides presenting different amino acid sequences as occur in the extracellular part of the Her2/neu protein. These peptides may be administered individually or in combination, in the form of multiple discrete peptides, each preferably conjugated separately to a delivery system. In yet another example, WO 2011/020604 describes fusion peptides comprising multiple Her2/neu B cell epitopes coupled to a virosome delivery system. These virosomes were shown to a induce a higher antibody titre against a single B cell epitope as compared to the same fusion peptides formulated with Montanide™ or an ISCOM-based delivery system.
Despite several attempts to develop a suitable vaccine for inducing immunity against Her2/neu, there is still no effective vaccine in clinical use. It is an aim of the present invention to provide an improve composition suitable for use as a vaccine for treating or preventing a condition such as cancer that is characterized by the expression or over-expression of Her2/neu.